Added method to compute a^x for double a and DerivativeStructure x.

This takes care of special cases like a=0, but encounters the same
limitation as rootN near the singularity 0^0. Combined derivatives like
order 2 or higher with respect to canonical variables give NaN and not
+/-infinity. First derivative with respect to a single variable works
well and provided the correct infinity.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/math/trunk@1517379 13f79535-47bb-0310-9956-ffa450edef68